1. Field of the Invention:
The subject invention relates to a method and apparatus for detecting the position of a movable object in a liquid-filled tank. In particular, the invention is a method and apparatus for accurately detecting the position of a movable object, such as a cleaning or inspection vehicle, which is submerged in a liquid-filled tank, such as an oil tank or the like.
2. Description of the Related Art:
It is well known to utilize acoustic methods for detecting the position of a floating vessel. For example, U.S. Pat. No. 4,905,210, which is incorporated herein by reference, discloses a system wherein a plurality of hydrophones are placed around the perimeter of a liquid impoundment. Acoustic travel time from the hydrophones to a transmitter located on a vessel floating in the impoundment is measured and the position of the floating vessel is determined by the intersection of arcs respectively defined by the distances which correspond to the measured travel times.
Also, it is known to determine the position of an underwater object utilizing acoustic techniques. For example, U.S. Pat. No. 3,928,840, which is incorporated herein by reference, discloses a system wherein three hydrophones are moored underwater and acoustic travel times from the hydrophones are continuously measured so as to determine the position of the object through a triangulation technique.
Also, the wide use of petroleum products, petrochemicals and various other liquid chemicals has required that large metal or other thin-walled storage tanks be constructed so as to store the liquids before distribution. Often these tanks are relatively complicated devices having pumps, fill pipes, ventilation systems, roof support systems, and the like. Also, these tanks are very large, often several hundred feet in diameter. Sediment must be periodically removed from these tanks to avoid contamination of the liquid stored therein. In addition, these tanks must be inspected for flaws periodically to avoid very expensive and environmentally hazardous leaks. For example, American Petroleum Institute (API) standard 635 requires that all tanks storing hydrocarbons be periodically inspected for flaws that can lead to leakage. It is difficult and costly to drain, clean and purge these large tanks for cleaning or inspection.
In view of this, small inspection and cleaning vehicles have been developed which can enter a large tank through a manway or other opening. Such vehicles are often equipped with a video camera or ultrasonic inspection systems as well as a cleaning apparatus. For example, one such system is described in U.S. Pat. No. 5,205,174. These vehicles are remotely controlled to move along the bottom of the tank and have aided in compliance with standards such as API 635 to a great degree. However, the position of the vehicle in the tank must be accurately determined at all times or else it is difficult to accomplish a cleaning operation efficiently. Also, if defects are found in the tank, the precise position of the vehicle must be known in order to determine the position of the defect and remedy the situation. Finally, it is necessary to precisely navigate around pumps and fill pipes which are disposed inside the tank.
Techniques similar to those described above have been used for tracking the position of a submerged cleaning/inspection vehicle in a tank. However, these methods are not desirable for use in such a situation because of several drawbacks. In particular, the tanks are generally exposed to the sun, which causes temperature gradients to form within the liquid in the tank. In other words, the liquid in the tank which is closest to the side thereof which is exposed to the sun will be at a higher temperature than the liquid which is further from the exposed side. Such a temperature gradient will cause erroneous position measurements in an acoustic system because the acoustic velocity in fluids changes significantly with temperature. Agitating the fluid in the tank will help to eliminate temperature gradients but severely hinders the cleaning process because the sediment will be stirred up from the bottom of the tank. Also, known detection methods have required that transducers be placed in contact with the liquid. This is not practical in large tanks because it necessitates insertion of the transducers into the tank or the formation of holes in the sides of the tank. Finally, the geometry of tanks cause problems such as insonification or "ringing" and errors due to the reflection and transmission of sound by the sides of the tank and interference due to mechanisms and structures in the tank, such as pumps and fill pipes.